Simulation of degradation of semiconductor disconnectors

 

The institute cooperates with other universities and industrial partners in the project DriveBattery2015. The main goal of this projects is the development of intelligent concepts of interconnection and control of modular battery management systems for electrical vehicles in order to increase the efficiency and the security and to lower the costs of the systems. A part of this project deals with semiconductor disconnectors, and within this part our institute simulates the degradation of such devices.

The degradation corrupts the functional safety of such devices, therfore a more profound understanding of the basic physical processes which lead to the degradation may help to improve this safety. It is well known that hot carriers (HC) cause the degradation. When these hot carriers impact on the silicon-silicondioxode-interface in the device, they have a sufficient kinetic energy to enter the oxide. Therefore, space charge is collected at the interface. This process can continue until the fixed interface charges change the electric field in the device in such a way that it cannot be used anymore.

 

For a simulation of these HC-effects, it is mandatory to simulate the energy- and position-dependent distribution function of electrons and holes. This can be achieved by solving the Boltzmann transport equation (BTE). Simplifications of the BTE, like the drift-diffusion model, which is the standard model for industrial TCAD simulations, cannot give the energy-dependant distribution function, and thus they fail at the description of HC effects.

Our approach to solve the BTE deterministicially is based on the spherical harmonics expansion (SHE). The resulting system of equations is very large (up to about 50 million unknowns) because of the required number of grid points for a detailed resolution of a realistic power transistor and because of the wide energy range resulting from the high bias values.

Nevertheless, the system of equations can be solved in an efficient way with suitable stabilization and discretization schemes which have been developed at the institute. As many power transistors are working close to the avalanche breakdown voltage, this operation condition is of special interest. The numerical treatment of the instabilities associated with the breakdown is an additional challenge.

 

It is possible to participate in this work by writing a bachelor or master thesis. The theses can focus on the mathematical modeling or the programming of simulation software.